You mean in the sense of a de-facto reserve currency? Well, I expect that, given the dominance of the Iron Bank in Essosi finance, it’s probably the Braavosi currency (which annoyingly we don’t have a name for), at least in the Free Cities. On the other hand, we see golden honors circulating in Volantis, Slaver’s Bay, and Qarth, which suggests competing monetary blocs.
In terms of what the exchange rate is, it’s a bit hard to say. We do know that the golden dragon is much larger than the golden honor or the golden hands that House Gardener minted, due to the consolidation of the entire Westerosi continent and thus the Westerland’s vast gold reserves. On the other hand, Westeros has an undeveloped financial system, so it may be that while dragons are more valuable for their metallic content, Braavosi coins are easier to use in commerce in the east.
We disagree here. It’s clearly established during Arya’s stay in Braavos that Volantene honors are the most widely used of the Free City currencies.
‘The blind girl went to work in the dark, stripping the dead of boots and clothes and other possessions, emptying their purses and counting out their coins. Telling one coin from another by touch alone was one of the first things the waif had taught her, after they took away her eyes. The Braavosi coins were old friends; she need only brush her fingertips across their faces to recognize them. Coins from other lands and cities were harder, especially those from far away. Volantene honors were most common, little coins no bigger than a penny with a crown on one side and a skull on the other. Lysene coins were oval and showed a naked woman. Other coins had ships stamped onto them, or elephants, or goats. The Westerosi coins showed a king’s head on the front and a dragon on the back. (DwD, The Blind Girl)’
Volantis is rich and all Western ships heading east have to stop at the city to resupply while most Eastern traders drop their cargos off there and turn around. If the precious metal content of Volantene coins is stable and reliable then it would be one of the more convenient currencies for anyone engaged in Summer Sea trade, the Iron Bank included.
1. I interpret that sentence differently. Arya’s saying that of the non-Bravaosi coins, Volantene coins are the most common, not that the Volantene coins are the most common in the Free Cities.
2 If that was the case, it’s a major worldbuilding flaw on GRRM’s part. Either the Iron Bank is the largest and most dominant bank in Essos or the Volantene currency is dominant, but it shouldn’t be both. The Bank of Amsterdam did not make loans denominated in Byzantine solidi.